The present invention relates to a semiconductor laser and, more particularly, to a semiconductor laser device which oscillates with a single longitudinal mode and has a continuous wavelength tuning capability.
As well known in the art, a distributed feedback (DFB) or distributed Bragg reflector (DBR) semiconductor laser oscillates with a stable single longitudinal mode, even during high-speed modulation, utilizing wavelength selectivity of a diffraction grating which is provided in a semiconductor crystal. With such a capability, a DFB or DBR laser is expected to be useful as a light source for future optical coherent transmission systems as well as for long distance, large capacity optical fiber communications. In an optical heterodyne system, which is one of the optical coherent transmission systems, it is a primary requisite that at a receiving station a beat signal having a predetermined frequency be provided by interference between incoming signal light and light which is oscillated by a local oscillation light source, or local oscillator. To meet this requirement, the local oscillator must constantly follow the wavelength of signal light while maintaining a certain predetermined frequency difference. Therefore, a laser which is to serve as a local oscillator needs to oscillate with a single longitudinal mode and, yet, feature wavelength controllability, in particular continuous wavelength controllability.
Wavelength-controllable, or tunable, single-wavelength semiconductor lasers include a DBR laser which was reported in "Bragg Wavelength-Tunable DBR-DC-PBH LD" (in Japanese) 1984 National Convention (Record) of the Institute of Electronics and Communication Engineers of Japan, Part 4, Paper No. 1022. A schematic structure of this tunable DBR laser is shown in FIG. 2. As shown, an active layer 3 having a 1.3 .mu.m wavelength composition and a P-InGaAsP guide layer 2 having a 1.2 .mu.m wavelength composition are sequentially grown on an n-InP substrate 1. A diffraction grating 10 having a period of about 4000 A is provided only in that part 15 of the surface of the guide layer 2 which serves as a tuning region, while that part 16 of the guide layer surface which serves as an active region is left flat. Thereafter, a p-InP cladding layer 4 and a p.sup.+ -InGaAsP cap layer 5 are sequentially grown on the entire surface of the guide layer 2. Provided above the tuning region 15 and the active region 16 are a tuning electrode 6 and a laser driving electrode 7, respectively. The cap layer 5 is partly removed to define a groove 11 between the electrodes 6 and 7 which insures electrical isolation between the electrodes 6 and 7. An n-electrode 9 is provided on the n-side of the multi-layer semiconductor. The DBR LD having such a structure oscillates with a single longitudinal mode when a drive current I.sub.d is injected into the active region 16, the wavelength being tunable in response to a tuning current I.sub.t injected into the tuning region 15. This is derived from the fact that the current I.sub.t injected into the tuning region 15 increases the carrier density at the tuning region 15 side of the laser and, thereby, lowers the refractive index through the plasma effect. As a result, the Bragg wavelength, which is dependent upon the period of the diffraction grating 10, is shifted toward shorter wavelength. Therefore, injection of the current I.sub.t causes the oscillation wavelength to vary toward shorter wavelength. However, the tuning characteristic particular to this kind of DBR LD involves mode jumping. Specifically, the DBR LD is incapable of performing continuous wavelength tuning, although it is capable of controlling the Bragg wavelength by means of the tuning current I.sub.t. The DBR LD cannot control the phase of light which, with the active region 16 viewed from the boundary between it and the tuning region 15, is propagated through the active region 16 and then returned by reflection from the right end of the LD which has an end facet.